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Genetic Engineering and Working with DNA

The technique illustrated in this animation produced by WGBH and Digizyme, Inc., shows how scientists use natural processes and technological innovations to insert genes into loops of DNA called plasmids. Plasmids can then be introduced into bacterial or other cells, which will proceed to replicate the inserted genes or induce the cells to produce such valuable proteins as human insulin and growth hormone. This resource is part of the Biotechnology collection.

A primary objective of biological science is to understand how living things function—how cells grow and reproduce, how plants absorb nutrients and make food, and how human digestion and metabolism convert that food to allow our own bodies to grow and reproduce. Biotechnology harnesses our understanding of how living systems function and uses that knowledge and modern technology to create beneficial products for society. Its approach addresses the function of cells and organisms at the level of genes and the proteins they encode.

The ability to alter genes inside organisms began in the early 1970s with the research of scientists Stan Cohen and Herb Boyer. Cohen studied plasmids—tiny rings of DNA that occur naturally inside bacterial cells. He was interested in plasmids because some of them carry genes that make bacterial cells resistant to antibiotics—and can transfer those genes to other bacteria. Although plasmids are separate from a bacteria's chromosomal DNA, they too are replicated, or copied. Thus, a bacterial cell may contain many copies of a plasmid. This means that a plasmid gene that codes for antibiotic resistance or any other trait will be copied and passed on from one generation of bacteria to another, where it will continue to produce that trait.

By the early 1970s, Cohen and his team had found a way to insert free-floating plasmids into E. coli bacterial cells. If there were a way to insert a new gene into plasmids, Cohen suspected that bacterial cells could serve as living copy machines, replicating the inserted gene and its trait for as long as the colony of bacterial cells survived.

Herb Boyer, who was also interested in E. coli bacteria, was studying an enzyme that cut strands of DNA at a particular location, or sequence. These so-called restriction enzymes had likely evolved as the bacterial cells' defense against foreign DNA, such as that introduced by viruses. For a geneticist like Boyer, however, restriction enzymes provided a way to cut plasmids and genes in specific locations. More importantly, some restriction enzymes cut DNA in such a way as to produce “sticky” ends, which enabled the plasmids and cut DNA fragments to bond easily to one another. To make this bond permanent, Boyer and his team used an enzyme called DNA ligase, which is a naturally occurring enzyme that cells use to repair DNA or to rejoin DNA molecules during cell division.

Together, Cohen and Boyer now had the natural mechanisms and technological innovations necessary to insert into bacterial plasmids genes not normally found there, and then to introduce those plasmids into cells. The bacteria did the rest. If a gene for human insulin were introduced, the cells produced insulin. If a gene for human growth hormone were introduced, the bacteria generated growth hormone. The era of biotechnology and genetic engineering had begun.